The invention relates to a heat exchanger, particularly an oil cooler, having a core provided with passages for a liquid to be cooled and passages for a cooling medium, and having a first collection tank for supplying the liquid and a second collection tank for withdrawing the same.
Heat exchangers of this type are known in many different embodiments and, in particular, in connection with motor vehicles (e.g. DE 297 19 311 U1). The liquid to be cooled is typically water or oil.
In addition, heat exchangers of this type are used to cool, for example, the oil of motors or compressors that are primarily operated in the open air and may be exposed to a wide range of ambient temperatures, for example, 55xc2x0 C. to xe2x88x9240xc2x0 C., depending on application and location. The object is to cool the oil, which is delivered at a temperature of perhaps 150xc2x0 C., by about 40-50xc2x0 C., i.e., to a temperature of approximately 100 to 110xc2x0 C. Thus, the cooling capacity must be selected in such a way that the desired cooling of the oil is reliably ensured even at the highest possible outside temperature. The disadvantage, however, is that the cooling capacity of said cooler is significantly overdimensioned at temperatures far below the freezing point and the oil is cooled more than necessary.
A further problem is due the fact that the viscosity of the typically used oils strongly depends on the temperature so that the flow properties of the oil are not optimal over the entire indicated temperature range. Since the oils becomes increasingly thick with decreasing temperatures, its flow resistance in the oil-carrying passages of the heat exchanger continues to increase with increasing cold. The flow resistance may become so great that the heat exchanger is completely or partially destroyed, particularly if the oil passages are equipped with turbulators that are intended to introduce turbulence into the flowing oil to enhance heat exchange.
Since heat exchangers of the type initially described are to be produced and sold irrespective of the climatic conditions in which they are used, attempts have been made to prevent said problem by connecting a thermostat-controlled bypass line in parallel, similar to motor vehicle radiators, to take up the oil flow as long as the oil is at a temperature below its operating temperature, whereas after reaching the operating temperature, the oil is guided through the heat exchanger. This measure alone is not sufficient, however, because it does not take into consideration the fact that the oil does not only become thicker at low temperatures but may even gel. Thus, if the operating conditions are such that the oil is cooled significantly at the instant when the bypass line is disconnected, the oil my gel, particularly in those passages where the cooling effect is particularly good. The flow rate of the oil then decreases significantly and drops practically to zero, while the flow rate in those passages where cooling is less effective is affected to a lesser extent. The resulting temperature differences in the heat exchanger have the effect that the hotter parts of the heat exchangers expand more than the cooler parts, which causes thermal stresses that gradually damage the structure of the walls defining the passages and ultimately cause cracks therein. As a result, the heat exchanger leaks and becomes unusable.
Finally, attempts have been made to prevent this problem in providing the heat exchanger with louvers that may be rolled up and down or by regulating the heat exchanger capacity. But such measures have thus far proven to be unsatisfactory because of their susceptibility to failure and their considerable cost.
It is, therefore, an object underlying this invention to design a heat exchanger mentioned above such that its cooling capacity can be adapted to the ambient temperatures.
A further object of this invention is to provide the heat exchanger mentioned above with simple and inexpensive means which makes possible to adapt the cooling capacity to the ambient temperature in a manner to counterbalance the oil gelling problems.
Yet another object of this invention is to design the heat exchanger such that destructions of the heat exchanger at very low temperatures as a result of congealing problems of the liquid to be cooled are largely prevented.
These and other objects of this invention are solved by a heat exchanger of the type mentioned above and having a core which is divided at least in two sections through which the liquid successively flows and which are interconnected by at least one third collection tank, wherein one of the sections is connected with the first collection tank and another one of the sections is connected with the second collection tank. The other section is also connected in parallel with a bypass line, which is connected with the third collection tank and can be connected or disconnected by means of a valve.
The invention is based on the idea of operating the heat exchanger at full capacity and with the entire usable length of the existing oil passages only at temperatures above a critical value. If the temperature is below a critical value, the bypass line is connected in order to take a section of the oil passages largely out of operation, reduce the cooling surface and decrease the capacity of the heat exchanger to a value sufficient for lower outside temperatures. If the outside temperatures rise again, the bypass line is disconnected again.
The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.